Ординатура / Офтальмология / Английские материалы / Eye Movements A Window on Mind and Brain_Van Gompel_2007

In Experiment 1, performance for inconsistent targets was facilitated during a brief presentation of a line drawing image. Although this facilitation was extinguished with the inversion of the images (Experiment 2), we cannot conclude that the facilitation was generated by the immediate detection of an inconsistent object in the scene because independent, naïve participants often failed to correctly identify the targets in question, even during unlimited scene viewing. If the effect was not mediated by the successful identification of the inconsistent targets, then the facilitation cannot be due to the perceived discrepancy between the target’s identity and the scene context.

This concern motivated the design of the more recognisable and naturalistic photographs used in Experiment 3, which failed to produce reliable evidence of consistency effects. The line drawings derived from them also failed to elicit any consistency effects (Experiment 4). Although the manipulation of consistency with these stimuli was less strong than with the line drawings used in Experiment 1, as all items could be found in a house and therefore were unusually, rather than impossibly, located, participants reliably rated the inconsistent targets as more unlikely than their matched consistent controls (Gareze, 2003). Therefore, the absence of a consistency effect could be due to an insufficiently strong consistency manipulation, indicating that the effect is unlikely to occur under naturalistic circumstances, or due to the improved recognisability of inconsistent targets, rendering them less salient in extrafoveal vision.

Previous work by Hollingworth and Henderson (1998, 1999, 2000) and Gordon (2004) directly contradicts the results of these experiments. In order to reconcile the two, it is necessary to consider the differences between these experiments, which may modulate such a discrepancy. To begin with, one of the major concerns the current experiments aim to address is the repeated display of the same or similar scenes, often up to eight times each. In these experiments, the number of times a participant viewed each scene background was strictly limited to prevent any learning effects. It was hypothesised that as inconsistent objects are better represented in memory, it would be plausible for an inconsistent target viewed in one background to influence performance on a subsequent repeated presentation. As the appropriate detection of scene gist is a crucial component of any evidenced consistency effects, the continued presentation of the same scene would be worth avoiding.

The importance of fixation position relative to the target has also been underestimated in previous work. Most experiments have involved a central fixation position with targets appearing at differing eccentricities within a scene, which does not consider the possibility that semantic information may be processed differently, if at all, in foveal and extrafoveal vision. Gordon (2004) presented targets at two eccentricities (near and far) but these varied across scenes. In one experiment, the mean near eccentricity was 2 9 but the range of eccentricities (0 8 –5 3 ) overlapped with that of far eccentricities (3 4 –8 8 ), which had a mean of 6 7 . Additionally, although object size was recorded, it was not manipulated systematically. Targets of approximately 2 square in size were presented at the above eccentricities, making many of the displays foveal or near foveal in presentation. In this way, it was impossible to distinguish between possible foveal and extrafoveal consistency effects.

Similarly, although object size was recorded, there was no analysis of whether object size affected performance on the task or any evident consistency effects. The results of the current series of experiments suggest that other object variables, such as object size, may modulate the expression of consistency effects. Significant and sometimes opposing consistency effects were found with data sets which showed no overall effect of the consistency manipulation, suggesting that the closer analysis of object-specific visual features may be valuable in the study of consistency effects.

A further concern relating to previous work which we have attempted to address is the nature of the visual stimuli used in these experiments. While many experiments have used line drawings derived from real-world scenes, few have used photographs of actual scenes. Our attempt to develop a set of naturalistic visual scenes resulted in a less extreme consistency manipulation than that found in the more frequently used stimuli. However, the failure to elicit consistency effects with these images calls into question the applicability of these effects to real-world viewing. Although some significant effects were found for certain target object sizes, the specificity of the conditions in which they occurred argues against the common occurrence of these effects during everyday scene viewing.

The investigation of eye movement behaviour also failed to provide evidence of differential extrafoveal processing for consistent and inconsistent targets. The only significant differences were found in fixation measures, confirming that inconsistent objects are fixated for longer than consistent ones (e.g. Henderson et al., 1999; De Graef et al., 1990). Even the line drawings, which exhibited an inconsistent object advantage in extrafoveal vision in Experiment 1, failed to produce any evidence of the earlier fixation of inconsistent targets. Despite previous findings of a reliable advantage for inconsistent objects presented extrafoveally in brief presentations paradigms (e.g. Gordon, 2004; Hollingworth & Henderson, 1998, 1999, 2000), no similar effect was found using these stimuli, suggesting that the effect may be difficult to elicit under more natural viewing conditions.

Acknowledgements

We are grateful to Peter De Graef for permission to use these scenes from the set available at ftp://michotte.psy.kuleuven.ac.be/pub/line_drw/ and for suggesting the manipulation in Experiment 2. We are also grateful to Lora Findlay for creating the stimuli used in Experiment 4 using Adobe Photoshop.

References

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PART 8

EYE MOVEMENTS IN NATURAL ENVIRONMENTS

Edited by

ROGER P. G. VAN GOMPEL

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Abstract

How do the limitations of attention and working memory constrain acquisition of information in the context of natural behavior? Overt fixations carry much information about current attentional state, and are a revealing indicator of this process. Fixation patterns in natural behavior are largely determined by the momentary task. The implication of this is that fixation patterns are a learnt behavior. We review several recent findings that reveal some aspects of this learning. In particular, subjects learn the structure and dynamic properties of the world in order to fixate critical regions at the right time. They also learn how to allocate attention and gaze to satisfy competing demands in an optimal fashion, and are sensitive to changes in those demands. Understanding exactly how tasks exert their control on gaze is a critical issue for future research.

A central feature of human cognition is the strict limitation on the ability to acquire visual information from the environment, set by limitations in attention. Related to this are the limits in retaining this information, set by the capacity of working memory. We are far from understanding how the organization of the brain leads to these limitations. We also have little understanding of how they influence the way that visual perception operates in the natural world, in the service of everyday visually guided behavior. Consideration of how the limited processing capacity of cognition influences acquisition of visual information leads us to the problem of how such acquisition is controlled. It is not really possible to address the question of precisely what information is selected from the image, and when it is selected, in the context of traditional experimental paradigms, where the trial structure is designed to measure a particular visual operation over repeated instances, each of short duration. In natural behavior, on the other hand, observers control what information is selected from the image and when it is selected. By observing natural behavior, knowledge of the task structure often allows quite well constrained inferences about the underlying visual computations, on a time scale of a few hundred milliseconds.

1. Eye movements and task structure

How can we study the acquisition of information in the natural world? Although incomplete, eye movements are an overt manifestation of the momentary deployment of attention in a scene. Covert attentional processes, of course, mean that other information is processed as well, but overt fixations carry a tremendous amount of information about current attentional state, and provide an entrée to studying the problem (Findlay & Gilchrist, 2003). Investigation of visual performance in natural tasks is now much more feasible, given the technical developments in monitoring eye, head, and hand movements in unconstrained observers, as well as the development of complex virtual environments. This allows some degree of experimental control while allowing relatively natural behavior. In natural behavior, the task structure is evident, and this allows the role of individual fixations to be fairly easily interpreted, because the task provides an external referent for the internal computations. In contrast, when subjects simply passively view images, the experimenter often has little control of, and no access to, what the observer is doing. When viewing pictures, observers may be engaged in object recognition, remembering object locations and identity, or performing some other visual operation. Immersion in a real scene probably calls for different kinds of visual computations, because observers may be interacting with the objects in the scene. When viewing images of scenes, some regularities in fixation patterns can be explained by image properties such as contrast or chromatic salience. However, these factors usually account for only a modest proportion of the variance (Itti & Koch, 2001; Mannan, Ruddock & Wooding, 1997; Parkhurst, Law, & Neibur, 2002).

Over the past ten years, a substantial amount of evidence has accumulated about deployment of gaze during ongoing natural behavior. In extended visuomotor tasks such as driving, walking, sports, playing a piano, hand-washing, and making tea or sandwiches,